This invention relates to fluidized bed combustion and in particular to an apparatus, system and process wherein the fluidization of the bed is achieved by mechanical means.
A fluidized bed results when a fluid, usually a gas, flows upward through a bed of suitably sized, solid particles at a velocity sufficiently high to buoy the particles, to overcome the influence of gravity, and to impart to them an appearance of great turbulence, similar to that of a violently boiling liquid. McGraw-Hill Encyclopedia of Science and Technology, Vol. 5, p. 346 (1966). Fluidized bed combustion has been found useful in the production of chemical reactions such as the oxidation of coal to produce heat for power generation and for process heat as well as in the disposal of combustible wastes through incineration.
In the conventional fluidized bed combustion system, the fluidized bed is produced by suspending the bed particles in a vertically rising stream of moving gas, such as air. The air moves fast enough to cause the particles to become suspended, but not entrained. The air-particle mixture takes on the characteristics of a "fluid". In a steam generation system in which the fluidized bed is utilized to burn coal, air is introduced at the bottom of a combustion chamber through a distributor plate which generally defines the lower boundary of the combustion chamber and contains perforations for passage of the air. Coal or other solids are fed into the combustion chamber and fluidized by the air which is injected in quantities and at pressures to produce the necessary fluidization while also providing combustion air. Such fluidized bed combustion systems generally provide a well mixed medium for efficient combustion and good heat transfer.
The prior art describes various and differing attempts at increasing the overall efficiency of fluidized bed combustion systems and overcoming process problems to enhance their operability and reliability. For example, U.S. Pat. No. 3,702,595, Muirhead et al., discloses a variation on a fluidized bed incinerator in which differential fluidization of the bed is brought about. That is, bed material.in one region is fluidized to a greater degree of agitation than that at another region by either varying the size of the apertures in the air distributor plate or utilizing two separate air boxes positioned beneath the plate. Similarly, U.S. Pat. No. 3,776,150, Evans et al., discloses a fluidized bed system for incineration of solid wastes having two chambers with fluidizing gas inlets designed so that the fluidized bed in the first chamber is more highly fluidized than the fluidized bed in the second chamber.
U.S. Pat. No. 4,066,024, O'Connor, discloses what is said to be a rotating fluidized bed combustor in which a heated fluidized bed is formed by rotating a body of sand in a horizontal cylindrical drum. The sand is lifted and dropped from the bed through an open portion of the drum which is heated. The sand is carried up one side of the rotating drum by scoop-like baffles to an approximate surface angle of 30.degree. whereupon it tumbles back producing a continuous rolling motion of the bed said to be not unlike the fluidizing effect achieved by forcing air or other gas up through a sand bed. The patent states that air may be introduced up through the bed to aid in combustion and that when such is done, the effect is virtually identical to a conventional fluidized bed incinerator although the air flow, not being needed for the fluidizing action, may be substantially less. Despite the statements in the patent, the "tumbling" of sand "downhill" as shown in its FIGS. 3-5 does not appear to provide a true fluidizing effect wherein the particles would posess turbulent motion.
U.S. Pat. No. 4,154,585, Melcher et al., relates to fluidized bed collectors used to remove fine ash, smoke and/or dust from combustion gases. In the system there disclosed, a grid used to impose an electric field and to polarize particles of the bed also performs as a distributor plate to support bed particles. Rappers are attached to such distributor plate and an apertured distributor plate to dislodge particles that may become attached to them.
Apart from fluidization, combustion apparatus have utilized apertured support surfaces to support a fuel bed, such as coal particles, and through which combustion air passes. These apertured plates, commonly called "grates", are frequently fitted with motor driven vibration generators to feed and move the fuel bed over the grate surface which may also be inclined to the horizontal. Vibration is generally for short intervals and controlled automatically in response to the fuel load demand.
An example of an oscillating grate is described in U.S. Pat. No. 1,945,782, Jones. In that patent there is shown a grate which supports a fuel bed and through which air and/or steam passes. In order to prevent the concentration of heat in a central zone, the grate includes an inner conical section which is adapted to rotate within an outer circumferential zone. The apex of the cone is made eccentric with respect to the axis of rotation so that agitation may be imparted to the central portion of the fuel bed. It is said that the fuel bed receives a wave-like undulating movement and that the introduction of steam thereunder prevents clogging of openings in the grate.
Thus, prior art attempts at improving fluidized bed combustion systems have been diverse and largely directed to specific modifications. In essentially all instances such variations have nevertheless retained air or gas as the true fluidizing source for the bed. Thus, such prior art systems continue to possess the requirements associated with the use of air or/gas as a fluidizing source, including the need to take precautions against undesired entrainment of particles in the air and/or gas stream, gas tight construction to accomodate the quantities and pressures required for fluidization and compromise in the control of firing rates and combustion air qualities as well as the limitation of efficient combustion to units of specific sizes.